Ocean Mixed Layer Research Articles

A Positive Low Cloud–Sea Surface Temperature Feedback in the East Asian Marginal Seas during El Niño Mature Winters and Their Following Spring

Abstract During the winter and subsequent spring of an El Niño year, the East Asian marginal sea (EAMS) exhibits positive sea surface temperature anomalies (SSTAs) and fewer low clouds, while the western North Pacific experiences negative SSTAs. In this study, we suggest that the positive SSTAs in EAMS are maintained by a positive low cloud–SST feedback. In neutral winters and springs, the EAMS is covered by low clouds, which have a cooling effect on surface temperatures. During an El Niño year, a western North Pacific anomalous anticyclone is established, and along its northwestern flank, there are favorable conditions for convergence of moisture and weaker surface latent heat flux over the EAMS. Once a positive SSTA has been established, a further reduction of turbulent mixing results in less low cloud and enhanced solar heating of the ocean mixed layer; this reinforces and maintains both the positive SSTA and the lack of low cloud via a positive feedback mechanism. The concurrent increase of low cloud–SST feedback and anticyclone circulation strengths is evident in the coupled-model simulations from phase 6 of the Coupled Model Intercomparison Project. Furthermore, sensitivity experiments, performed with the atmospheric components of Community Earth System Model (CESM2), reveal that a positive SSTA helps to maintain the western North Pacific anomalous anticyclone. Four pacemaker-coupled experiments by CESM2, with sea surface temperature in the equatorial Pacific restored to the observational anomalies plus the model climatology and altered low cloud feedback over EAMS, suggest that the low cloud–SST feedback results in more than the maintenance of a positive SSTA over the EAMS: the positive feedback is also a previously overlooked mechanism for the maintenance of the western North Pacific anomalous anticyclone. Significance Statement The East Asian marginal sea (EAMS) and western North Pacific are important areas that bridge El Niño and the climate of East Asia. Unlike the cold sea surface temperature anomaly (SSTA) over the western North Pacific during El Niño, the positive SSTA over EAMS, which is covered by winter low cloud, has received less attention. We suggest that a “low cloud–SST” feedback—namely, one in which decreasing low-level clouds allows more sunlight to strike the ocean surface and favors higher SST—maintains the positive SSTA over EAMS. We also configure a widely used atmospheric model with a set of preset SSTA patterns that mimic different climate patterns. Our experiments with different climate patterns and CMIP6 historical runs show that the low cloud–SST feedback (through the positive SSTA) is a possible supplementary mechanism for reinforcing the WNP anomalous anticyclone.

Changes in the annual sea ice freeze–thaw cycle in the Arctic Ocean from 2001 to 2018

Abstract. The annual sea ice freeze–thaw cycle plays a crucial role in the Arctic atmosphere—ice–ocean system, regulating the seasonal energy balance of sea ice and the underlying upper-ocean. Previous studies of the sea ice freeze–thaw cycle were often based on limited accessible in situ or easily available remotely sensed observations of the surface. To better understand the responses of the sea ice to climate change and its coupling to the upper ocean, we combine measurements of the ice surface and bottom using multisource data to investigate the temporal and spatial variations in the freeze–thaw cycle of Arctic sea ice. Observations by 69 sea ice mass balance buoys (IMBs) collected from 2001 to 2018 revealed that the average ice basal melt onset in the Beaufort Gyre occurred on 23 May (±6 d), approximately 17 d earlier than the surface melt onset. The average ice basal melt onset in the central Arctic Ocean occurred on 17 June (±9 d), which was comparable with the surface melt onset. This difference was mainly attributed to the distinct seasonal variations of oceanic heat available to sea ice melt between the two regions. The overall average onset of basal ice growth of the pan Arctic Ocean occurred on 14 November (±21 d), lagging approximately 3 months behind the surface freeze onset. This temporal delay was caused by a combination of cooling the sea ice, the ocean mixed layer, and the ocean subsurface layer, as well as the thermal buffering of snow atop the ice. In the Beaufort Gyre region, both (Lagrangian) IMB observations (2001–2018) and (Eulerian) moored upward-looking sonar (ULS) observations (2003–2018) revealed a trend towards earlier basal melt onset, mainly linked to the earlier warming of the surface ocean. A trend towards earlier onset of basal ice growth was also identified from the IMB observations (multiyear ice), which we attributed to the overall reduction of ice thickness. In contrast, a trend towards delayed onset of basal ice growth was identified from the ULS observations, which was explained by the fact that the ice cover melted almost entirely by the end of summer in recent years.

Typical and anomalous pathways of surface-floating material in the Northern North Atlantic and Arctic Ocean

Surface waters of the oceans carry large amounts of material, including sediment grains, plankton organisms, and ice crystals, as well as pollutants, e.g., oil and plastic. Transport and spatio-temporal distribution of this material depend on its properties and on the dynamical processes in the ocean mixed layer—currents, waves, turbulence, and convective mixing—acting at a wide range of scales. Due to its importance for marine physics, biogeochemistry and ecology, substantial research efforts have been invested in recent years in observations and modelling of ocean material transport, especially in the context of marine plastic pollution. Nevertheless, many important questions remain unanswered. In this work, numerically simulated trajectories of surface-floating particles in the period 1993–2020 are used to analyse typical and anomalous transport pathways in the northern North Atlantic and the Arctic Ocean. Model validation is performed based on additional simulations of 387 buoy tracks from the International Arctic Buoy Programme in the years 2014–2020. The trajectories are computed based on surface currents from a hydrodynamic model and Stokes drift from a spectral wave model. It is shown that due to high amplitudes of Stokes drift (comparable with wind-induced currents in ice-free parts of the domain of study), combined with high directional variability, the drifting paths are substantially modified in ice-free regions, underlying the important role of wave-induced currents in surface material transport. A statistical analysis of sim 1.6, 10^8 trajectories reveals patterns of connections between nearshore locations in the domain of study, the associated drift times and path sinuosity. Seasonal variability of transport, which differs between the Arctic Ocean and the North Atlantic, is found for typical transport routes following the larger-scale circulation patterns. Crucially, in both sub-domains episodic, but very strong transport events between otherwise isolated locations occur, associated with anomalous atmospheric circulation and, arguably, providing ‘windows of opportunity’ for dispersal of various organisms to new locations. It is shown for two examples in the North Atlantic region that an unusual combination of atmospheric circulation indices explains the anomalous transport, thus providing a predictive tool for future events. In the Arctic, analogous phenomena are modified by the state of the sea ice cover.

Similarity of the turbulent kinetic energy dissipation rate distribution in the upper mixed layer of the tropical Indian Ocean

Turbulence within the upper ocean mixed layer plays a key role in various physical, biological, and chemical processes. Between September and November 2011, a dataset of 570 vertical profiles of the turbulent kinetic energy (TKE) dissipation rate, as well as conventional hydrological and meteorological data, were collected in the upper layer of the tropical Indian Ocean. These data were used to statistically analyze the vertical distribution of the TKE dissipation rate in the mixed layer. The arithmetic-mean method made the statistical TKE dissipation rate profile more scattered than the median and geometric-mean methods. The statistical TKE dissipation rate were respectively scaled by the surface buoyancy flux and the TKE dissipation rate at the mixed-layer base. It was found that the TKE dissipation rate scaled by that at the mixed-layer base exhibited better similarity characteristics than that scaled by the surface buoyancy flux, whether the stability parameter D/|LMO| was greater or less than 10, indicating that the TKE dissipation rate at the mixed-layer base is a better characteristic scaling parameter for reflecting the intrinsic structure of the TKE dissipation rate in the mixed layer, where D and LMO are respectively the mixed-layer thickness and the Monin-Obukhov length scale. The parameterization of the TKE dissipation rate at the mixed-layer base on the shear-driven dissipation rate and the surface buoyancy flux was further explored. It was found that the TKE dissipation rate at the mixed-layer base could be well fitted by a linear combination of three terms: the wind-shear-driven dissipation rate, the surface buoyancy flux, and a simple nonlinear coupling term of these two .

Global Marine Heatwaves and Cold‐Spells in Present Climate to Future Projections

AbstractMarine heatwave (MHW) characteristics, such as total days, average duration, and mean intensity, have increased in most global oceans. In contrast, marine cold‐spell (MCS) characteristics have a decreasing trend over most regions. However, the difference between MHW and MCS in the mean and trend in the present and future climate is unclear. Here, we provide a comparative global assessment of these changes based on satellite sea surface temperature for 1982–2020 and climate model projections for 2021–2070. Globally, there are clear differences in the mean and trend between MHW and MCS metrics. On seasonal timescales, MHWs mainly occur in summer and autumn, while MCSs generally occur in winter and spring. The occurrence area of MHWs shows an increasing trend during 1982–2020, while the occurrence area of MCSs shows an opposite trend. Furthermore, the increasing trend in MHW average duration led to a near‐permanent MHW status in the future projections. Upper‐ocean warming due to anthropogenic forcing is closely related to increase in MHWs and decrease in MCSs. In addition, the shallower ocean mixed layer depth in the future is favorable for the occurrence of MHWs. In contrast, the projected weakening of the Atlantic meridional overturning circulation facilitates the occurrence of MCSs in the subpolar North Atlantic. These results provide a global baseline for the difference and relationship between MHW and MCS under present climate and future projections.

Dominant Coupling Mode of SST in Maritime Continental Region and East Asian Summer Monsoon Circulation

AbstractUsing a series of data from the National Oceanic and Atmospheric Administration (NOAA) and singular value decomposition (SVD) method, the dominant coupled mode between sea surface temperature (SST) anomaly in Maritime Continent (MC) and circulation anomaly in East Asia in summertime and its underlying mechanism are revealed. Results show that the first SVD mode is characterized by the in‐phase variations of SST anomaly in the entire MC and sea level pressure (SLP) anomaly in East Asia, and their co‐variation is measured by an SVD joint index, . Based on composite difference fields, the ocean mixed layer heat budget reveals that the maintenance of the MC regional SST anomaly is jointly determined by ocean dynamics and heat flux. The anomalous SST warming near Java Island is dominated by the meridional advection of ocean current, while the SST warming in the Bay of Bengal, the South China Sea and the Northwest Pacific are mainly contributed by the increased downward latent heat flux and shortwave radiation flux. The SLP anomaly pattern in East Asia coupled with SST anomaly in the MC region is represented by the anomalous anticyclone in the Northwest Pacific, which can be maintained by the ‐related atmospheric vertical cell. The ‐related circulation anomalies tend to favor the summer precipitation in the middle and lower reaches of the Yangtze River and the south of Yunnan‐Guizhou Plateau, but lead to negative precipitation anomaly in central and Northeast China.

Evaluating the effects of a symmetric instability parameterization scheme in the Xisha-Zhongsha waters, South China Sea in winter

As one of the important submesoscale instabilities, symmetric instability (SI) widely exists in the ocean surface mixed layer (SML), which enhances the vertical material transport in the SML and also the exchanges between the SML and the ocean interior. Due to the small spatial scales of SI, O (10 m–1 km), which are not resolved by most current ocean models, the application of SI parameterization is an alternative choice in the coming decades to include the SI effects in ocean models and improve the model performance. In this study, we evaluate the impacts of SI in a realistic configuration with the SI parameterization scheme applied in the Xisha-Zhongsha waters, South China Sea in winter by using the Coastal and Regional Ocean Community Model (CROCO) version of the Regional Ocean Modeling System. Compared to the SI-lacking case, the SI energy source, the geostrophic shear production, is increased and elimination of anticyclonic potential vorticity is revealed in the SI-parameterized case. According to the energy analysis, multi-scale interactions are also influenced by the SI. The effective wind energy input is reduced, and the potential energy release in the SML is suppressed. Moreover, the SI scheme makes the SML depth shallower and closer to the reanalysis one. This work demonstrates a good performance of the SI scheme applied in regional models in representing SI effects.

Improvement of the Ocean Mixed Layer Model via Large-Eddy Simulation and Inverse Estimation

Abstract The ocean mixed layer model (OMLM) is improved using the large-eddy simulation (LES) and the inverse estimation method. A comparison of OMLM (Noh model) and LES results reveals that underestimation of the turbulent kinetic energy (TKE) flux in the OMLM causes a negative bias of the mixed layer depth (MLD) during convection, when the wind stress is weak or the latitude is high. It is further found that the entrainment layer thickness is underestimated. The effects of alternative approaches of parameterizations in the OMLM, such as nonlocal mixing, length scales, Prandtl number, and TKE flux, are examined with an aim to reduce the bias. Simultaneous optimizations of empirical constants in the various versions of Noh model with different parameterization options are then carried out via an iterative Green’s function approach with LES data as constraining data. An improved OMLM is obtained, which reflects various new features, including the enhanced TKE flux, and the new model is found to improve the performance in all cases, namely, wind-mixing, surface heating, and surface cooling cases. The effect of the OMLM grid resolution on the optimal empirical constants is also investigated. Significance Statement This work illustrates a novel approach to improve the parameterization of vertical mixing in the upper ocean, which plays an important role in climate and ocean models. The approach utilizes the data from realistic turbulence simulation, called large-eddy simulation, as proxy observation data for upper ocean turbulence to analyze the parameterization, and the statistical method, called inverse estimation, to obtain the optimized empirical constants used in the parameterization. The same approach can be applied to improve other turbulence parameterization, and the new vertical mixing parameterization can be applied to improve climate and ocean models.

Numerical Investigation of Tropical Indian Ocean Barrier Layer Variability

AbstractThe barrier layer (BL) is part of the surface ocean isolated from the local air‐sea exchange processes despite it's great potential to store the heat beneath the mixed layer. The tropical oceans are known for the presence of BL. The interannual variability of the BL of the tropical Indian Ocean is examined in this study with a concomitant analysis of observations and outputs from an ocean general circulation model. The model faithfully reproduces the tropical variability of the dynamics and thermodynamics of the Indian Ocean when compared with observations and other studies. The BL thins (thickens) in the eastern (central‐to‐western) tropical Indian Ocean during the positive phase of the Indian Ocean Dipole Mode (IOD). The individual roles of five major forcing mechanisms on BL‐IOD coupling are examined. The thinning in the east is caused by upward heaving thermocline due to upwelling Kelvin waves during the positive phase of the IOD, causing the isothermal layer to shoal steeper than the mixed layer. The surface freshwater fluxes substantiate the thinning of BL in the east during this time, while the net heat fluxes thin the BL in the entire tropical Indian Ocean. The basin‐wide salinity (temperature) dynamics have widespread and complementary impacts, a BL thickening (thinning) in the equatorial and eastern tropical Indian Ocean during positive IOD years, with the former overwhelming the latter. The thickening of BL in the central‐to‐western tropical Indian Ocean during positive IOD is due to a combined action of salinity and freshwater forcing. The thermocline heaving, temperature and salinity dynamics and buoyancy forcing could explain the net variability of BL in the tropical Indian Ocean almost entirely in the model. The ocean‐only response shows that a BL weakens the shoaling (deepening) of the ocean mixed layer during positive (negative) IOD years in the eastern tropical Indian Ocean. As barrier layers store heat beneath the mixed layer, their role in Indian Ocean weather and climate needs to be examined with coupled model experiments.

Surface net heat flux estimated from drifter observations

The ocean mixed layer temperature equation is used to estimate the surface net heat flux from drifter measurements. The net heat flux is determined for both the climatologic and tropical cyclone (TC) conditions. The spatial distributions of the drifter-derived heat fluxes under both the two conditions are similar to those derived from satellite observations. However, the drifter-derived climatologic heat flux appears to be weaker in magnitude than that derived from satellites, and performs better in closing the energy budget with a global mean value of 3.9 W m−2. The drifter-derived heat flux also performs better than the satellite-derived heat flux under TCs, using the buoy observations as a reference considering metrics such as the meen error, mean absolute error, root mean-square error and percent bias. The spatially averaged mean net heat flux derived from drifters under TCs is −124 W m−2 at 10° N, and decreases to −85 W m−2 at 30° N, however, these values are much larger than those obtained from satellites (−63 W m−2 and −21 W m−2, respectively). As additional components for the mixed layer temperature equation, both the entrainment velocity and eddy diffusivity in climatology show large amplitudes in regions with strong currents such as the Western Boundary Current and Antarctic Circumpolar Current. However, under TC conditions large values of the entrainment velocity and eddy diffusivity mostly appear in regions with strong winds.

Targeted artificial ocean cooling to weaken tropical cyclones would be futile

Proposals to use technology to cool sea surface temperatures have received attention for the potential application of weakening a tropical cyclone ahead of landfall. Here, application of an ocean-mixing aware maximum potential intensity theory finds that artificial ocean cooling could drastically weaken tropical cyclones over high sea surface temperature and deep ocean mixed layer environments, especially for fast storm motion speeds. In contrast, realistic mesoscale numerical simulations reveal that massive regions - the largest evaluated here contains a volume of 2.1 × 104 km3 and a surface area of 2.6 × 105 km2 - of artificially cooled ocean waters could weaken a tropical cyclone two days before landfall by 15% but only under the most ideal atmospheric and oceanic conditions. Thus, the fundamental theory provides an unreachable upper-bound that cannot be attained even by expending vast resources.

Controls on nitrite oxidation in the upper Southern Ocean: insights from winter kinetics experiments in the Indian sector

Abstract. Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season. Because this nitrate constitutes a regenerated rather than a new nutrient source to phytoplankton, it will not support the net removal of atmospheric CO2. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis–Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115±2.3 to 245±18 nM, suggesting that nitrite-oxidizing bacteria (NOB) require a minimum (i.e., “threshold”) nitrite concentration to produce nitrate. The half-saturation constant for nitrite oxidation ranged from 134±8 to 403±24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150 nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two- to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. Additionally, nitrite oxidation may be limited by dissolved iron, the availability of which is low across the upper Southern Ocean. Our findings have implications for understanding the controls on nitrification and ammonium and nitrite distributions, both in the Southern Ocean and elsewhere.

A Potential Energy Analysis of Ocean Surface Mixed Layers

AbstractTurbulent mixing in the ocean surface boundary layer leads to the presence of a surface mixed layer. This mixed layer is important for many phenomena including large‐scale ocean dynamics, ocean‐atmosphere coupling, and biological and biogeochemical processes. Analysis of the ocean mixed layer requires one to estimate its vertical extent, for which there are various definitions. Correspondingly, there are uncertainties on how to best identify an ocean surface mixed layer for a given application. We propose defining the mixed layer depth (MLD) from energetic principles through the potential energy (PE). The PE based MLD is based on the concept of PE anomaly, which measures the stratification of a layer of seawater by estimating its energetic distance from a well‐mixed state. We apply the PE anomaly to diagnose the MLD as the depth to which a given energy could homogenize a layer of seawater. We evaluate the MLD defined by common existing methods and demonstrate that they contain a wide range of PE anomalies for the same MLD, particularly evident for deep winter mixed layers. The MLD defined from the PE anomaly ensures a more consistent MLD identified for a large range of stratifications. Furthermore, the PE method relates to the turbulent kinetic energy budget of the ocean surface boundary layer, which is fundamental to upper ocean mixing processes and parameterizations. The resulting MLD is more representative of active boundary layer turbulence, and is more robust to small anomalies in seawater properties.

Role of Sea Surface Physical Processes in Mixed-Layer Temperature Changes During Summer Marine Heat Waves in the Chile-Peru Current System.

We identified anomalously warm sea surface temperature (SST) events during 1980–2019 near the major upwelling center at Punta Lavapié in the central Chile‐Peru Current System, using the European Centre for Medium‐Range Weather Forecasts reanalysis and focusing on time scales of 10 days to 6 months. Extreme warm SST anomalies on these time scales mostly occurred in the austral summer, December through February, and had spatial scales of 1000s of km. By compositing over the 37 most extreme warm events, we estimated terms in a heat budget for the ocean surface mixed layer at the times of strongest warming preceding the events. The net surface heat flux anomaly is too small to explain the anomalous warming, even when allowing for uncertainty in mixed‐layer depth. The composite mean anomaly of wind stress, from satellite ocean vector wind swath data, during the 37 anomalous warming periods has a spatial pattern similar to the resulting warm SST anomalies, analogous to previous studies in the California Current System. The weakened surface wind stress suggests reduced entrainment of cold water from below the mixed layer. Within 100–200 km of the coast, the typical upwelling‐favorable wind stress curl decreases, suggesting reduced upwelling of cold water. In a 1000‐km area of anomalous warming offshore, the typical downwelling‐favorable wind stress curl also decreases, implying reduced downward Ekman pumping, which would allow mixed‐layer shoaling and amplify the effect of the positive climatological summertime net surface heat flux.

Effects of symmetric instability in the Kuroshio Extension region in winter

Submesoscale symmetric instability (SI) in the ocean surface mixed layer (SML) plays a significant role in the forward energy cascade and vertical material transports. Due to its small spatial scales of typically less than 1 km, SI is not resolved by current climate ocean models and most regional models. This work investigates the SI effects in the Kuroshio Extension (KE) region by using a SI parameterization scheme implemented in the Coastal and Regional Ocean Community Model (CROCO). Compared to a SI-lacking model at the same spatial resolution of 500 m, a SI-parameterized model more closely resembles an ultra-high resolution (20 m) non-hydrostatic model that is intended to resolve SI directly on the basis of evaluating the energy source of SI: geostrophic shear production. As SI restratifies the SML, the SI-parameterized model simulates a shallower SML depth in the KE region compared to the SI-lacking one, which is closer to the observed state. The potential vorticity is also modulated and a reduction of negative PV is found due to the parameterized SI. The results here demonstrate the capability of the SI scheme in the representation of the SI impacts and an improvement in the simulation of the KE.

Pathways to Better Prediction of the MJO: 2. Impacts of Atmosphere-Ocean Coupling on the Upper Ocean and MJO Propagation.

This study investigates effects of atmosphere‐ocean coupling on MJO precipitation and eastward propagation, and upper ocean conditions during and after MJO passage. To explore pathways for improving MJO prediction, three model experiments are conducted using the Unified Wave Interface‐Coupled Model at convection‐permitting (4 km) resolution: (a) uncoupled atmosphere‐only, (b) coupled atmosphere‐ocean, and (c) coupled atmosphere‐ocean with improved air‐sea flux algorithm simulations. The model simulations are compared with observations from the DYNAMO field campaign in 2011. Both coupled atmosphere‐ocean simulations produced eastward propagation of the MJO where the uncoupled, atmosphere‐only simulation did not. The uncoupled model overestimates both precipitation and surface winds associated with the MJO, while coupled model simulations substantially reduce model bias. Improved air‐sea fluxes lead to systematic improvements in precipitation, winds, sea surface temperature, and the ocean mixed layer when compared to the original coupled simulation. This leads to further improvement of the MJO's eastward propagation speed compared with observations. Despite these improvements, the regional coupled simulations still have difficulty representing the extent of convectively suppressed conditions in the Indian Ocean after MJO passage, which indicates the importance of the large‐scale environment from lateral boundary conditions. Coupled model simulations also reveal some issues in the representation of upper ocean stratification in the ocean model, especially errors in salinity, which result in overestimation of the mixed layer depth after MJO passage.

Impact of Rainfall on Tropical Cyclone‐Induced Sea Surface Cooling

AbstractTropical cyclones (TCs) are often accompanied by strong winds and torrential rains. While the winds associated with TCs tend to enhance mixing in the upper ocean, the freshwater input from rain can stratify the water column and limit mixing. However, the extent to which the stabilizing effect of rainfall can compete with wind‐induced mixing and to what degree it modulates TC‐induced sea surface cooling remain unknown. Here we show, using a suite of observations, that heavy rains under weak TCs can significantly reduce the magnitude of cold wakes induced by them. When compared to storms with low rain rates, the ocean surface under TCs with high rain rates freshens significantly and cools less. High‐resolution climate model simulations and idealized experiments with an ocean mixed layer model support these results and reveal that oceanic mixing processes are primarily responsible for reduced cooling under TCs, with a lesser role for surface fluxes.

Subpolar Atlantic Ocean mixed layer heat content variability is increasingly driven by an active ocean

Cold conditions in the upper layer of the subpolar North Atlantic Ocean, at a time of pervasive warming elsewhere, have provoked significant debate. Uncertainty arises both from potential causes (surface heat loss and ocean circulation changes) and characteristic timescales (interannual to multidecadal). Resolution of these uncertainties is important as cold conditions have been linked to recent European weather extremes and a decline in the Atlantic overturning circulation. Using observations, supported by high resolution climate model analysis, we show that a surprisingly active ocean regularly generates both cold and warm interannual anomalies in addition to those generated by surface heat exchange. Furthermore, we identify distinct sea surface temperature patterns that characterise whether the ocean or atmosphere has the strongest influence in a particular year. Applying these new insights to observations, we find an increasing role for the ocean in setting North Atlantic mixed layer heat content variability since 1960.

Description of historical and future projection simulations by the global coupled E3SMv1.0 model as used in CMIP6

Abstract. This paper documents the experimental setup and general features of the coupled historical and future climate simulations with the first version of the US Department of Energy (DOE) Energy Exascale Earth System Model (E3SMv1.0). The future projected climate characteristics of E3SMv1.0 at the highest emission scenario (SSP5-8.5) designed in the Scenario Model Intercomparison Project (ScenarioMIP) and the SSP5-8.5 greenhouse gas (GHG) only forcing experiment are analyzed with a focus on regional responses of atmosphere, ocean, sea ice, and land. Due to its high equilibrium climate sensitivity (ECS of 5.3 K), E3SMv1.0 is one of the Coupled Model Intercomparison Project phase 6 (CMIP6) models with the largest surface warming by the end of the 21st century under the high-emission SSP5-8.5 scenario. The global mean precipitation change is highly correlated with the global temperature change, while the spatial pattern of the change in runoff is consistent with the precipitation changes. The oceanic mixed layer generally shoals throughout the global ocean. The annual mean Atlantic meridional overturning circulation (AMOC) is overly weak with a slower change from ∼ 11 to ∼ 6 Sv (Sverdrup) relative to other CMIP6 models. The sea ice, especially in the Northern Hemisphere, decreases rapidly with large seasonal variability. We detect a significant polar amplification in E3SMv1.0 from the atmosphere, ocean, and sea ice. Comparing the SSP5-8.5 all-forcing experiment with the GHG-only experiment, we find that the unmasking of the aerosol effects due to the decline of the aerosol loading in the future projection period causes transient accelerated warming in the all-forcing experiment in the first half of the 21st century. While the oceanic climate response is mainly controlled by the GHG forcing, the land runoff response is impacted primarily by forcings other than GHG over certain regions, e.g., southern North America, southern Africa, central Africa, and eastern Asia. However, the importance of the GHG forcing on the land runoff changes grows in the future climate projection period compared to the historical period.

Cause of the Intense Tropics-Wide Tropospheric Warming in Response to El Niño

Abstract During El Niño events, a strong tropics-wide warming of the free troposphere is observed (of order 1 K at 300 hPa). This warming plays an important role for the teleconnection processes associated with El Niño but it remains unclear what initiates this warming. Since convective quasi-equilibrium only holds in regions of deep convection, the strong free-tropospheric warming implies that the warmest surface waters (where atmospheric deep convection occurs) must warm during El Niño. We analyze the evolution of the oceanic mixed layer heat budget over El Niño events as function of sea surface temperature (SST). Data from the ERA5 and an unforced simulation of a coupled climate model both confirm that SSTs during an El Niño event increase at the high end of the SST distribution. The data show that this is due to an anomalous heat flux from the atmosphere into the ocean caused by a decrease in evaporation due anomalously weak low-level winds (i.e., relative to the wind speed observed in the domain of deep convection in the climatological base state). It is hypothesized that the more zonally symmetric circulation during El Niño is responsible for the weakening of low-level winds. The result of a substantial heat flux into the ocean in the domain of atmospheric deep convection (the opposite of the canonical heat flux out of the ocean into the atmosphere observed in the cold eastern Pacific) caused by a decrease in low-level wind speed implies that the prominent tropospheric warming results from mechanical forcing.